With the use of the rat yolk sac as an in vitro model system, it has been observed that glucagon markedly inhibits pinocytosis. Epinephrine has a similar but lesser effect, which is augmented by theophylline. Ascorbic acid partially reverses the glucagon inhibition. The hypothesis that these effects are mediated by the known influence of cyclic AMP on microtubular function is supported by a similar inhibition produced by colchicine. On the other hand, neither dibutyryl cyclic AMP or dibutyryl cyclic GMP per se added to the incubation medium affected pinocytosis in either control or glucagon-treated preparations. It is proposed to measure directly the levels of cyclic nucleotides in control and glucagon-treated preparations and in addition to examine them electron microscopically in an effort to detect alterations in microtubular status. The ability of cyclic GMP-elevating agents, such as carbachol, to reverse the inhibition and any concomitant morphological effects will also be tested.